1. Field of the Invention
The invention is in the field of devices called microbolometers, and specifically, is a microbolometer for detecting photons in the infrared, ultraviolet, EUV, and X-ray ranges.
2. Description of the Prior Art
Several currently available systems are used to detect single photons in the infrared, ultraviolet, EUV, and X-ray ranges. A Transition Edge Sensor based (TES) microbolometer is described in U.S. Pat. No. 5,880,468 to Irwin and in A. J. Miller et al, xe2x80x9cTransition Edge Sensors as Single Photon Detectorsxe2x80x9d, IEEE Transactions on Applied Superconductivity, Vol 9, No. 2, pages 4205-4208, June 1999. The TES based microbolometer consists of 3 separate parts: a metallic absorber, which transforms the energy of the single photon into heat in a normal-metal base layer, which transmits the heat to the thermometer; and the thermometer (the TES) itself.
Current TES systems have four inherent problems limit their effectiveness. First, four electric leads are needed for each pixel, making it difficult to arrange a large number of pixels close together in an array. Secondly, the sensors are currently placed on membranes, and sometimes even kept out of contact with the substrate by a pop up arrangement, which makes the system vulnerable to mechanical damage. Thirdly, the fraction of the focal plane area devoted to the absorber is small, so the spatial efficiency of the devices is limited. In addition, power must be continuously supplied to the TES for the read-out to occur and to keep the TES substantially warmer than the substrate if event rates of even 1 kHz are to be achieved. Fourthly, the operational temperature is very low, typically bias temperatures are below 100 xcexcK. For these reasons, many important practical imaging applications are difficult to achieve.
Other devices for detecting photons include cooled integrating Charge Coupled Device (CCD) digital imaging systems. Although CCDs have better focal plane area utilization than TES systems, and are available in large array format, CCDs have very limited energy resolution capability. Modern semiconducting CCD detectors have already achieved efficiencies in excess of 90% of their theoretical limit with excellent spatial resolution, but are limited in their temporal resolution by the long read-out time per pixel. More importantly, their non-dispersive spectral resolution is limited statistically so that xcex94EFWHM cannot exceed the level of about 100 eV for E=6 KeV photon events.
The other main class of detector, superconducting tunnel junctions (STJ), are not bolometers and have never functioned as well as the bolometers, especially at high energy ranges (X-ray). Examples of STJs are described in P. Verhoeve, N. Rando, A. Peacock, A. van Dordrecht, M. Bavdaz, J. Verveer, D. J. Goldie, M. Richter, and G. Ulm, Proc. Int. Workshop LTD-7 (Ed. S. Cooper), Munich, 1997, pp. 97-100.
Energy-dispersive broadband detectors which are easily integrated into a detector array are desired for both space research and laboratory instrumentation. The next generation of single-photon detectors will need to be hyperspectral imaging detectors capable of obtaining high spectral resolutions (up to E/xcex94EFWHM=10,000 for E=1-6 KeV x-ray photon events and xcex/xcex94xcex greater than 100 for UV photons) and processing high event rates, while maintaining high spatial resolution and high focal plane efficiency. For hyperspectral imagers, a new class of detectors is needed. Thermoelectric hot-electron microcalorimeters have been proposed for use as photon detectors (D. Van Vechten, K. S. Wood, G. G. Fritz, A. L. Gyulamiryan, V. Nikogosyan, N. Giordano, T. Jacobe, and A. M. Gulian, xe2x80x9cThermoelectric Single-Photon Detectors: Isotropic Seebeck Sensorsxe2x80x9d, 18th International Conference on Thermoelectrics, pp. 477-480 (1999)), and G. G. Fritz, K. S. Wood, D. Van Vechten, A. L. Gyulamiryan, A. S. Kuzanyan, N. J. Giordano, T. M. Jacobs, H. D. Wu, J. S. Horwitz, A. M. Gulian, xe2x80x9cThermoelectric single-photon detectors for X-ray/UV radiationxe2x80x9d; X-Ray and Gamma-Ray Instrumentation for Astronomy XI, Proc. SPIE, Vol 4140, (2000), the disclosures of which are incorporated herein by reference.
Photon detectors using materials which are strongly thermally anisotropic have been proposed in A. M. Gulian, D. Van Vechten, K. S. Wood, G. G. Fritz, J. S. Horwitz, M. S. Osofsky, J. M. Pond, S. B. Qadri, R. M. Stroud, J. B. Thrasher, xe2x80x9cImaging Detectors Based on the Response of Anisotropic Layered Materialsxe2x80x9d, IEEE Trans. Applied Superconductivity, Vol. 9, No. 2, pp. 3194-3197, (1999) and in D. Van Vechten, K. S. Wood, G. G. Fritz, J. Horwitz, A. L. Gyulamiryan, A. Kuzanyan, V. Vartanyan, and A. M. Gulian, xe2x80x9cImaging Detectors based on anisotropic thermoelectricityxe2x80x9d, Nuclear Instruments and Methods in Physics Research section A, 444 (2000)42-45 both of which are incorporated by reference. Advantages of these novel approaches are numerous.
An object of the invention is to provide a photon detector which requires no applied voltage or current for sensor operation.
An object of the invention is provide a photon detector with high photon efficiency and high spectral resolution, with an improved temporal response.
An object of the invention is to provide a photon detector with sufficiently fast temporal response time to detect photons arriving at a rate of 1,000,000 per second in each detector unit.
Another object of the invention is to provide a spatially efficient photon detector where the absorbers intercept a large fraction of the incident photons which is easily and efficiently integrated into a photon detector array.
In accordance with these and other objects which will become apparent, the invention described herein is a fast photon detector with high energy and position resolution, which may be used in the infrared, ultraviolet, EUV, and X-ray ranges. An absorber receives a photon and transforms the energy of the photon into a change in temperature within the absorber. A thermoelectric sensor is thermally coupled to the absorber. When the absorber receives the photon, the energy of the photon is very quickly transformed into a time dependent temperature difference across the sensor. A heat sink is thermally coupled to the sensor, to maintain the heat flow across the sensor. The absorber, sensor, and heat sink are disposed upon a dielectric substrate, such that the heat transfer from the sensor to the dielectric substrate is much slower than the signal duration. Superconducting leads are used to measure the voltage which develops across the sensor in response to a photon event. The superconducting leads may be attached to the input coil of a SQUID flux transformer circuit.
In one embodiment of the invention, a superconducting bridge is disposed upon the substrate between the absorber and the heat sink, at a distance from the contact with the sensor. The superconducting bridge allows the absorber-sensor-heat sink-superconductor to act as a current loop, which generates a measurable flux.
In another main embodiment, an isotropic, thin superconducting oxide film is disposed upon a dielectric substrate. The superconducting oxide film acts as a thermoelectric sensor, and absorbs photons. A large voltage response across the longitudinal direction of the sensor results from the temperature gradient between the top of the sensor and the dielectric substrate, which acts as a heat sink. In another embodiment, an absorber and an optional insulating layer are disposed upon and thermally coupled to the thin semiconducting oxide film to ensure complete absorption of all incident photons.